The present invention relates to a clamping device and more particularly, a clamping device for holding workpieces.
Sheet clamps or expandable-mandrel-type devices are utilized for temporarily holding sheets of material together, such as metal panels joined by riveting. These clamping devices have found particular application in the aircraft industry, where overlapping skin panels are temporarily secured together to maintain them in a rivet-hole-aligned position prior to riveting. The clamps frequently include elongated, slidable, needle-like structures with enlarged heads, which are introduced through a pair of aligned rivet holes. The sheets are secured by forced separation of the enlarged heads.
Clamping action for many of the clamping devices is achieved by pulling the needles toward the clamp body over a spreader bar, which causes transverse separation of the needle ends. Some of these clamp-like devices utilize a spring-biased action with the needle structures normally withdrawn into a clamp body, and the enlarged heads are in proximity to the clamp body leading end for subsequent extension and insertion into the sheet materials. The clamps are operable by extending the needles over and beyond a spreader bar, which allows the flexible head ends to be moved toward each other for insertion into aligned rivet holes. The clamp body is subsequently brought into proximity to the workpiece surface, either by the operator or by reaction to the spring pulling the needle members into the clamp body during needle member withdrawal through the clamp body. The spreader bar forces the needle ends to move apart to engage the inner panel, inner surface, thus securing the workpieces between the clamp body and the enlarged ends of the needles. Indicative of this normally-secured biasing action are the clamping devices shown in U.S. Pat. Nos. 2,188,450; 2,271,012; 2,365,787; 2,463,731; and 3,196,734. These clamps thus require a mechanical force to extend the needle members for insertion into the aligned holes of the sheets to be joined, and subsequent release of the moving mechanical force withdraws and expands the needle heads to contact the underlying workpiece inner surface and retain the workpieces between the expanded head ends and the clamp body. However, the maximum clamping force which the clamps exert is in some cases limited by the spring force, that is the product of the spring constant and the spring displacement. This spring force may not be adequate to clamp the two sheets together. The mechanical force to extend the needle members may be manually applied either by hand or with a tool to overcome a larger clamp bias force.
Other clamping or fastening devices utilize a mechanical operator to both engage and disengage the enlarged ends of needle-like clamps. These clamping devices with mechanical operators may utilize hydraulic fluids rather than a spring biasing force.